diff --git a/composer.json b/composer.json
index b5550de..5cd4f06 100644
--- a/composer.json
+++ b/composer.json
@@ -11,6 +11,7 @@
     }
   ],
   "require": {
+    "php": ">=7.1",
     "indieweb/link-rel-parser": "0.1.*",
     "mf2/mf2": ">=0.3.2"
   },
@@ -25,7 +26,7 @@
     }
   },
   "require-dev": {
-    "phpunit/phpunit": "^10 || ^11"
+    "phpunit/phpunit": "^9.6 || ^10 || ^11"
   },
   "scripts": {
     "test": "phpunit"
diff --git a/src/p3k/HTTP/Guard.php b/src/p3k/HTTP/Guard.php
index cb7e05c..38dbfff 100644
--- a/src/p3k/HTTP/Guard.php
+++ b/src/p3k/HTTP/Guard.php
@@ -14,6 +14,18 @@ class Guard {
   // read as an IP address. Refuse them rather than guess which one.
   const NUMERIC_HOST = '/^(0x[0-9a-f]*|[0-9]+)(\.(0x[0-9a-f]*|[0-9]+))*\.?$/i';
 
+  // Addresses that are not on the public internet: the IANA IPv4 and IPv6
+  // special-purpose registries, plus multicast. Listed here rather than
+  // left to FILTER_FLAG_GLOBAL_RANGE, which needs PHP 8.2, so that every
+  // PHP version refuses the same addresses.
+  const NON_PUBLIC = [
+    '0.0.0.0/8', '10.0.0.0/8', '100.64.0.0/10', '127.0.0.0/8', '169.254.0.0/16',
+    '172.16.0.0/12', '192.0.0.0/24', '192.0.2.0/24', '192.88.99.0/24', '192.168.0.0/16',
+    '198.18.0.0/15', '198.51.100.0/24', '203.0.113.0/24', '224.0.0.0/4', '240.0.0.0/4',
+    '::/128', '::1/128', '::ffff:0:0/96', '64:ff9b:1::/48', '100::/64', '2001::/23',
+    '2001:db8::/32', '3fff::/20', '5f00::/16', 'fc00::/7', 'fe80::/10', 'fec0::/10', 'ff00::/8',
+  ];
+
   private $_allow_hosts = [];
   private $_allow_cidrs = [];
   private $_resolver;
@@ -90,9 +102,14 @@ class Guard {
 
   /** Whether an address is on the public internet, looking inside IPv6 forms that carry an IPv4 address. */
   public static function is_public($address) {
-    if(!filter_var($address, FILTER_VALIDATE_IP, FILTER_FLAG_GLOBAL_RANGE))
+    if(!filter_var($address, FILTER_VALIDATE_IP))
       return false;
 
+    foreach(self::NON_PUBLIC as $cidr) {
+      if(self::in_cidr($address, $cidr))
+        return false;
+    }
+
     $embedded = self::_embedded_ipv4($address);
     if($embedded !== null)
       return self::is_public($embedded);
@@ -125,13 +142,13 @@ class Guard {
     return self::is_public($address);
   }
 
-  // NAT64 (64:ff9b::/96, 64:ff9b:1::/48) and 6to4 (2002::/16) addresses
-  // reach an IPv4 address, which must be public too.
+  // NAT64 (64:ff9b::/96) and 6to4 (2002::/16) addresses reach an IPv4
+  // address, which must be public too.
   private static function _embedded_ipv4($address) {
     $bin = @inet_pton($address);
     if($bin === false || strlen($bin) !== 16)
       return null;
-    if(self::in_cidr($address, '64:ff9b::/96') || self::in_cidr($address, '64:ff9b:1::/48'))
+    if(self::in_cidr($address, '64:ff9b::/96'))
       return inet_ntop(substr($bin, 12, 4));
     if(self::in_cidr($address, '2002::/16'))
       return inet_ntop(substr($bin, 2, 4));
diff --git a/tests/GuardTest.php b/tests/GuardTest.php
index c953cd7..efed7c7 100644
--- a/tests/GuardTest.php
+++ b/tests/GuardTest.php
@@ -11,7 +11,7 @@ class GuardTest extends TestCase {
   }
 
   public static function publicAddresses() {
-    return [['8.8.8.8'], ['93.184.216.34'], ['2606:4700::1'], ['2001:4860:4860::8888']];
+    return [['8.8.8.8'], ['93.184.216.34'], ['100.128.0.1'], ['198.20.0.1'], ['223.255.255.255'], ['2606:4700::1'], ['2001:4860:4860::8888'], ['64:ff9b::808:808'], ['2002:808:808::']];
   }
 
   public static function nonPublicAddresses() {
@@ -19,14 +19,19 @@ class GuardTest extends TestCase {
       ['127.0.0.1'], ['127.8.9.10'], ['10.11.11.80'], ['172.16.0.1'], ['192.168.1.1'], ['169.254.169.254'],
       ['100.64.0.1'], ['0.0.0.0'], ['::'], ['::1'], ['fe80::1'], ['fc00::1'], ['fd12:3456::1'],
       ['::ffff:127.0.0.1'], ['::ffff:10.0.0.1'], ['64:ff9b::7f00:1'], ['64:ff9b::a00:1'], ['2002:7f00:1::1'], ['2002:a9fe:a9fe::'],
+      ['192.0.0.8'], ['192.0.2.1'], ['198.18.0.1'], ['198.19.255.255'], ['198.51.100.7'], ['203.0.113.5'], ['224.0.0.1'],
+      ['239.255.255.250'], ['255.255.255.255'], ['100.127.255.255'], ['::ffff:8.8.8.8'], ['64:ff9b:1::1'], ['100::1'],
+      ['2001::1'], ['2001:db8::1'], ['3fff::1'], ['fec0::1'], ['ff02::1'], ['not an address'], [''],
     ];
   }
 
+  /** @dataProvider publicAddresses */
   #[\PHPUnit\Framework\Attributes\DataProvider('publicAddresses')]
   public function testPublicAddresses($address) {
     $this->assertTrue(Guard::is_public($address));
   }
 
+  /** @dataProvider nonPublicAddresses */
   #[\PHPUnit\Framework\Attributes\DataProvider('nonPublicAddresses')]
   public function testNonPublicAddresses($address) {
     $this->assertFalse(Guard::is_public($address));
@@ -42,6 +47,7 @@ class GuardTest extends TestCase {
     ];
   }
 
+  /** @dataProvider blockedUrls */
   #[\PHPUnit\Framework\Attributes\DataProvider('blockedUrls')]
   public function testBlockedUrls($url) {
     $result = self::guard(['localhost.example' => ['127.0.0.1'], 'mixed.example' => ['93.184.216.34', '10.0.0.1']])->check($url);